As an effective treatment method for coronary artery disease such as angina pectoris or myocardial infarction, there is a percutaneous coronary intervention (PCI). This is a treatment method performed using a catheter, and has become the mainstream of a coronary artery disease treatment after the 1990s due to an extremely low burden on a patient compared to that of other surgical procedures.
In the PCI, a mesh-like metallic tube called a stent, which is manufactured by performing laser processing on a thin, narrow tube, is very effective in securing blood flow in a narrowed or infarcted blood vessel, and is currently an essential device (PTL 1). A stent which is widely used in the PCI is made of SUS316L which is a material mainly used for medical purposes, tantalum, a cobalt alloy, nickel, a titanium alloy, or the like. However, since the stent is semipermanently placed in a treated vascular disease site, there is a problem of recurrence of stenosis and the like.
In order to solve this problem, “a stent decomposable and dissolvable in the body” is intensively examined, and for example, a stent base material made of a biodegradable polymer has been developed (NPL 1). However, as described in NPL 1, the biodegradable polymer has a weak “force to support a blood vessel”, and thus is insufficient to secure blood flow in a narrowed site. Therefore, a stent made of metal is more expected in terms of force to support a blood vessel than that made of polymer. Particularly, magnesium is originally present as a body element and has excellent biodegradability, and thus is a promising stent base material. In the present invention, a medical device represented by a stent base material having excellent biodegradability is called a biodegradable medical device.
Recently, a stent (mesh-like metallic tube) which employs a magnesium material as a base material has received attention all over the world as a next-generation minimally-invasive stent in which a burden on a patient is much smaller than that of an existing stent.
However, it is difficult to control time (biodegradation time) for the magnesium material to disappear in vivo compared to that of a polymer. In addition, the magnesium material has a hexagonal close-packed (hcp) structure, and thus a slip system during deformation at room temperature is limited only to (0001). Therefore, ductility is insufficient and breaking easily occurs in a stage of expanding a stent. In order to control the length of the biodegradation time and enhance ductility, a method of changing a material composition is well-known. However, a method of changing a material composition one by one has poor mass production efficiency.
In addition, since the magnesium material has a lower tensile strength than an iron-based material or an aluminum-based material and has a hexagonal close-packed (hcp) structure, a slip system during deformation at room temperature is limited only to (0001), and the workability is extremely insufficient.
Therefore, in the related art, it is difficult to obtain a long, thin, and narrow magnesium tube (round tube) having a diameter of 2 [mm] or less, a thickness of 200 [μm] or less, a length of 500 [mm] or greater, and a dimensional precision of 0.15[%] or less, which is necessary to process a stent. The magnesium round tube is produced by cutting or drawing a cast material or a material obtained by forging or extruding the cast material. However, the dimensional precision is poor, and the obtained length also does not satisfy 100 [mm].
As existing methods of processing a round tube, there are an extrusion method of extruding a billet into a hollow tube by using a mandrel, a seam welding method of performing cold winding on a plate into a cylindrical shape and welding butting portions, a drawing method (PTLs 2 and 3) of drawing a raw tube by using a die or a roll and reducing the cross-sectional area of the raw tube, and a deposition method or a sputter deposition method of directly coating a metal core with a round tube (PTL 4). However, the extrusion method and the seam welding method are limited to a case of manufacturing a round tube having a relatively large tube diameter from the viewpoint of process limitations, or the dimensions of a processing object, die strength, and the like, and thus cannot be applied to a case of manufacturing a thin, narrow tube. In a case of manufacturing a thin, narrow tube, the deposition method, the sputter deposition method, and the drawing method (PTL 3) are mainly used.
According to the deposition method and the sputter deposition method, an extremely thin, narrow tube having a tube diameter of 1 [mm] or less and a thickness of several tens of [μm] can be manufactured. However, it is difficult to process a long round tube due to limitations on the dimensions of the chamber and the metal core.
Hereinafter, a method of manufacturing a thin, narrow tube by using the drawing method will be described in more detail. In addition, the thin, narrow tube manufactured by using the drawing method is also called a drawn body.
The drawing method will be described with reference to FIGS. 13(a) to 13(d). The drawing method is broadly classified into four methods which are tube sinking, fixed plug drawing, floating plug drawing, and mandrel drawing. As shown in FIG. 13(a), the tube sinking is a method of drawing only a round tube 213 using a die 211. A roll may also be used instead of the die 211. During the tube sinking, as the process progresses, wrinkles are generated in an inner wall surface 213a of the round tube 213 and thus the thickness locally varies. Therefore, there is a problem in that the inner wall surface 213a cannot obtain dimensional precision.
As shown in FIG. 13(b), the fixed plug drawing is a method of allowing a plug 222B fixed to a support bar 222A to pass through an inside 223c of a round tube 223 and drawing the round tube by using the die 211 while supporting an inner wall surface 223a of the round tube with the plug 222B. A roll may also be used instead of the die 221. According to the fixed plug drawing, both the inner wall surface 223a and an outer wall surface 223b can obtain high dimensional precision. However, in a case where the round tube 223 to be processed is a narrow tube or a long tube, there is a problem in that it is technically difficult to manufacture the plug support bar 222A having a strength applicable to the case.
As shown in FIG. 13(c), the floating plug drawing is a method of allowing a plug 232 which is not fixed to pass through an inside 233c of a round tube 233 and drawing the round tube by using a die 231 while supporting an inner wall surface 233a of the round tube 233 with the plug 232. A roll may also be used instead of the die 231. Since a support bar that fixes the plug 232 thereto is not used, the method can be applied to a case where the round tube 233 to be processed is a narrow tube or a long tube. However, since the direction of the plug 232 is easily changed and the inner wall surface 233a of the round tube cannot be supported with a uniform force, the dimensional precision of the inner wall surface 233a is inferior to that of the fixed plug drawing. In addition, the processing force during the floating plug drawing is strong, and thus the round tube 233 may receive an excessive pressure during the processing and may be easily broken. In addition, in a case where the inner diameter of the round tube 233 is 1 [mm] or less, it is technically difficult to manufacture the plug 232 having a shape applicable to the case. Even when the plug can be manufactured, there is a problem in that handling is difficult.
As shown in FIG. 13(d), the mandrel drawing is a method of drawing a round tube 243 in a state where a mandrel 242 is inserted therethrough by using a die 241. A roll may also be used instead of the die 241. The mandrel drawing is a method applicable even when the round tube 243 is a narrow tube or a long tube. However, after the drawing, the mandrel 242 needs to be drawn out from the round tube 243. However, in a case where the round tube 243 is made of a material having a low strength, such as a magnesium material, when the mandrel 242 is drawn out, the entire round tube 243 may be deformed, and both an inner wall surface 243a and an outer wall surface 243b cannot obtain high dimensional precision. As a result, it is difficult to process the round tube 243 after the mandrel drawing into a desired mesh-like shape using a laser or the like in order to form a stent.